ValeyevK. G., and DzhalladovaI. A. 2003. “Derivation of Moment Equations for Solutions of a System of Nonlinear Difference Equations Dependent on a Semi-Markov Process”. Ukrains’kyi Matematychnyi Zhurnal 55 (6), 858-64. https://umj.imath.kiev.ua/index.php/umj/article/view/3960.